Use Conditional Compilation to Toggle Costly Debugging Features

You can insert code for debugging into a driver through conditional compiles by using a preprocessor symbol such as DEBUG or by using a global variable. With conditional compilation, unnecessary code can be removed in the production driver. Use a variable to set the amount of debugging output at runtime. The output can be specified by setting a debugging level at runtime with an ioctl or through a debugger. Commonly, these two methods are combined.

The following example relies on the compiler to remove unreachable code, in this case, the code following the always-false test of zero. The example also provides a local variable that can be set in /etc/system or patched by a debugger.

#ifdef DEBUG
/* comments on values of xxdebug and what they do */
static int xxdebug;
#define dcmn_err if (xxdebug) cmn_err
#else
#define dcmn_err if (0) cmn_err
#endif
/* ... */
    dcmn_err(CE_NOTE, "Error!\n");

This method handles the fact that cmn_err(9F) has a variable number of arguments. Another method relies on the fact that the macro has one argument, a parenthesized argument list for cmn_err(9F). The macro removes this argument. This macro also removes the reliance on the optimizer by expanding the macro to nothing if DEBUG is not defined.

#ifdef DEBUG
/* comments on values of xxdebug and what they do */
static int xxdebug;
#define dcmn_err(X) if (xxdebug) cmn_err X
#else
#define dcmn_err(X) /* nothing */
#endif
/* ... */
/* Note:double parentheses are required when using dcmn_err. */
    dcmn_err((CE_NOTE, "Error!"));

You can extend this technique in many ways. One way is to specify different messages from cmn_err(9F), depending on the value of xxdebug. However, in such a case, you must be careful not to obscure the code with too much debugging information.

Another common scheme is to write an xxlog() function, which uses vsprintf(9F) or vcmn_err(9F) to handle variable argument lists.